1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a secondary battery having a protrusion formed on a terminal plate of a cap assembly for increased contact resistance during contact with a negative electrode tab and for easy welding of the negative electrode tab.
2. Description of the Related Art
As portable wireless appliances including video cameras, portable telephones, and portable computers tend to have reduced weight while incorporating more functions, much research has been conducted on secondary batteries which are used as the driving power source thereof. For example, secondary batteries include nickel cadmium batteries, nickel hydrogen batteries, nickel zinc batteries, and lithium secondary batteries. Among them, lithium secondary batteries are widely used in the cutting-edge electronic appliance field because they can be recharged, they can be made in a compact size while having larger capacity, and they have high operating voltage and high energy density per unit weight.
A secondary battery is formed by placing an electrode assembly, which is composed of positive and negative secondary electrode plates and a separator, into a can together with an electrode and sealing a top opening of the can with a cap assembly.
The electrode assembly is formed by winding the positive and negative electrode plates with the separator interposed between them. The positive electrode plate has a positive electrode tab attached thereto while protruding from the upper end of the electrode assembly. The negative electrode plate has a negative electrode tab attached thereto while protruding from the upper end of the electrode assembly. The positive and negative electrode tabs are spaced a predetermined distance from each other in the electrode assembly for electrical insulation between them. The positive and negative electrode tabs are generally formed of a nickel-based metal.
The cap assembly includes a cap plate, an insulation plate, a terminal plate, and an electrode terminal. After being attached to a separate insulation case, the cap assembly is attached to the top opening of the can and seals the can.
The cap plate is formed of a metal plate having a size and a shape corresponding to those of the top opening of the can. The cap plate has a first terminal through-hole formed at the center thereof, into which the electrode terminal is inserted. When the electrode terminal is inserted into the first terminal through-hole, a tube-type gasket tube is attached to the outer surface of the electrode terminal and is inserted together, in order to insulate the electrode terminal and the cap plate from each other. The cap plate has an electrolyte injection hole formed on a side thereof. After the cap assembly is assembled to the top opening of the can, an electrolyte is injected through the electrolyte injection hole, which is then sealed by a separate sealing means.
The electrode terminal is connected to the negative electrode tab of the negative electrode plate or to the positive electrode tab of the positive electrode plate and acts as a negative or positive terminal.
The insulation plate is formed of an insulation material such as a gasket and is attached to the lower surface of the cap plate. The insulation plate has a second terminal through-hole formed thereon, which is positioned to correspond to the first terminal through-hole of the cap plate, so that the electrode terminal can be inserted therein. The insulation plate has a seating groove formed on the lower surface thereof with a size corresponding to that of the terminal plate so that the terminal plate can be seated thereon.
The terminal plate is formed of a nickel steel alloy (34-37% of Ni and balance of Fe), which is a nickel-based alloy, and is positioned on the lower surface of the insulation plate. The terminal plate has a third terminal through-hole formed thereon, which is positioned to correspond to the first terminal through-hole of the cap plate, so that the electrode terminal can be inserted therein. The terminal plate is electrically connected the electrode terminal while being insulated from the cap plate, because the electrode terminal is insulated by the gasket tube as it extends through the first terminal through-hole of the cap plate.
The negative electrode tab which is attached to the negative electrode plate is welded to a side of the terminal plate and the positive electrode tab which is attached to the positive electrode plate is welded to the other side thereof. The negative and positive electrode tabs can be attached by resistance welding or laser welding, the resistance welding being preferred.
According to a recent method for reducing the internal resistance of secondary batteries, each component is formed of a metal having a low electrical resistance. In other words, the internal resistance of secondary batteries is reduced to avoid electrical loss within the batteries. Although the terminal plate is formed of a nickel steel alloy which has a high resistance, it is replaced by a nickel-based metal which has a low resistance to avoid electrical losses caused by high resistance.
When the terminal plate is formed of a nickel-based metal, however, the contact resistance between the terminal plate and the negative electrode tab decreases and it becomes difficult to weld the negative electrode tab. As such, the weldability between the terminal plate and the negative electrode tab deteriorates and the welding tends to be performed between the negative electrode tab and the welding rod.